1. Field of the Invention
This invention relates to a method of removing noble metal contaminants from a mineral acid bath, and more particularly, in the manufacturing of packaged semiconductor devices, a method for removing contaminant Po-210 from a heated phosphoric acid bath using silicon as a getter.
2. Description of the Prior Art
In the process of manufacturing large scale integrated circuits (LSI) from a silicon substrate or wafer, phosphoric acid is often used as a solution to selectively remove silicon nitride.
Phosphoric acid commonly contains trace elements of Polonium-210 (Po-210). Po-210 is a radioactive element and a source of alpha radiation emissions. Levels of Po-210 and, in proportion, alpha emissions, vary depending on the content of Po-210 in phosphorous used to make the acid solution. The Po-210 present in any deposit of phosphorous is a function of the natural decay of U-238. Po-210 Levels may vary greatly from an imperceptible emission level to significant alpha emissions.
It is well known in the industry that alpha particle emissions are one of several known causes of soft errors in LSI memory devices. Soft errors have been defined to be random, non-recurring single bit errors in memory devices. They are not permanent, i.e., no physical defects are associated with the failed bit. A bit showing a soft error is completely recovered by the following write cycle, for example, in a dynamic memory device where refresh of memory stored data occurs every several nanoseconds.
Other identified causes of soft errors are system noise, voltage marginality, sense amplifiers and pattern sensitivity, all statistical predictors of the rate at which soft errors will occur (SER).
Po-210, when present in phosphoric acid in the etching process, has been identified as a source of alpha particle emissions. Po-210 has an affinity for Silicon (Si) in the acid bath and plates onto the surface of the Si wafers which are being etched for production of finished LSI memory and other devices. The Po-210 remains bonded at surface sites on the silicon wafer, through later subsequent manufacturing steps. The finished LSI circuit or die results in a memory device with internally emitted alpha particles. The alpha particles are emitted by the Po-210 contaminating the die. Thus, the memory device becomes itself a source of contribution of SER.